The nose gear, e.g., wheel steering system of an aircraft, is generally a single channel hydroelectric mechanism and has the potential to create a hazardous situation if certain failures occur during critical phase of aircraft operation. The electrohydraulic servo of the steering system includes a pedal position linear variable differential transducer (LVDT) which provides nose gear position command, a servo loop comprised of an electrohydraulic valve driven by a valve driver amplifier and a linear actuation position feedback transducer. Potential for failures exists in the servo loop. A wiring failure between the valve drive amplifier and the valve will result in a relatively slow drift of nose gear rotation because the current in both valve coils becomes zero, and control of the valve is lost. Certain failures in the valve drive amplifier such as a shorted or open transistor will result in a rapid nose gear rotation due to errant current in the valve coils. An open wire in the excitation wiring of the position feedback transducer will cause the nose gear to be difficult to control, since pilot inputs would control nose gear rate and not position. An open wire in the feedback transducer output will also cause a system failure and the input transducer will have no control. Failures occurring outside the servo loop are typically less hazardous. It is therefore desirable to provide a monitoring system to detect failures and automatically disengage the nose gear steering system.
It is therefore an object of the present invention to provide an improved monitoring circuit.
Another object of the invention is to provide a monitoring circuit utilizing a pair of simplified phase-sensitive full wave amplifying demodulators.
Still another object of the invention is to provide a system for monitoring the integrity of an aircraft nose gear steering system.